1. Field of the Invention
The present invention relates to a lens unit used in a camera or the like. More specifically, it relates to an apparatus for detecting position of an optical element constituting the lens, and to a lens unit including an apparatus for displaying distance information to the object and focal length information.
2. Description of the Related Art
In a camera having automatic focus adjustment mechanism, focus adjustment is automatically performed when the lens is directed to an object. There are various methods of focus adjustment of a lens. As an example, a lens unit is known in which part of optical elements constituting the lens, for example, a front lens group is moved along the optical axis based on focus detection information indicative of the state of defocus with respect to the object, so that in-focus state is realized. Since the distance information to the object is important data when taking a photograph, the distance information to the object is displayed on a lens barrel, for example, when the lens is in-focus with respect to the object.
In cameras in which focal length can be altered, the focal length is often displayed on the lens barrel, for example.
In the conventional lens unit, a movable lens barrel positioned in a fixed lens barrel is rotated around a helicoid screw, or operated along a cam, so as to perform focus adjustment operation and focal length changing operation. Therefore, distance to the object or the focal length information set in the lens unit has been displayed utilizing relative angle of movement between the fixed lens barrel and the movable lens barrel.
As for the method of moving the lens unit, another example includes impact type actuator method in which drive pulses are applied to a piezoelectric device, by which the lens is moved along the optical axis.
In a structure in which driving signal is output to the impact type actuator based on focus detection information or set information with respect to the object and part of the optical elements constituting the lens unit is moved along the optical axis, the movable lens barrel is not necessary as in the conventional lens unit, and only the lens support frame may be moved. However, in such a structure, information with respect to the lens cannot be displayed utilizing angle of relative movement between the fixed lens barrel and the movable lens barrel.
In the driving mechanism of optical elements to which the above described impact type actuator is applied, lens holding frame moves along a driving axis. Therefore, when a magnetized rod is provided parallel to the driving axis, a magneto-resistive element is attached to the lens holding frame and a position sensor (MR sensor) for detecting the position based on the change in the magnetic resistance is provided, the position of the lens holding frame, that is, the position of the moving lens may be detected. In that case, if the space between the magnetized rod and the magneto-resistive element varies, there would be error in detection. Therefore, the variation in the space between the magnetized rod and the magneto-resistive element must be within a prescribed tolerable range over considerably long distance of movement of the optical elements. For this purpose, it is necessary that the magnetized rod is positioned parallel to the driving axis and that swing of the lens holding frame during movement is prevented. These requirements make assembly of the lens unit quite difficult.
Other than magnetic type position sensor, an optical sensor employing a subscale and a main scale having small slits passing and intercepting light has been known. In either of the magnetic or optical sensor, sinusoidal wave is obtained as a detection signal. An intermediate level of the detection signal is compared with an analog signal represented by the sinusoidal wave, crossing of the signal with the intermediate level is converted to a pulse signal, and the number of pulse signals is counted to detect the amount of movement.
In order to increase resolution of detected amount of movement, the width of the slit of the scale may be made narrower in the optical system, and the width of magnetization of the magnet should be made thinner in the magnetic system.
However, the narrower the slit width of the scale or the thinner the width of magnetization, the smaller becomes the signal from the detector, and the poorer becomes the signal-to-noise ratio. Further, in the optical system, the gap between the main scale and the subscale becomes smaller, and in the magnetic system, the gap between the magnetized magnet and the magnetic detecting element becomes smaller, so that in either system, the width of acceptable variation becomes very small. As a result, very severe dimensional accuracy is required of the structure of the element and the scale, and from this point, resolution of detection is limited.
Therefore, apart from conversion of an analog signal to the digital signal at the intermediate point level, a method of interpolation has been known in which analog signals are divided into small pieces and digital signals are divided into small pieces so as to improve resolution in detection. Here, the term interpolation refers to the method of obtaining phase information finer than the signal frequency, by dividing periodical signal output.
However, the signal obtained in the detector is a sinusoidal wave, which includes linearly changing portion as well as a portion changing moderately with signal polarity being inverted. Therefore, resolution cannot be improved uniformly. A method to solve such a problem is disclosed, for example, in NIKKEI Mechanical , May 31, 1993. FIG. 30 is an illustration showing the method of interpolation disclosed in NIKKEI Mechanical . Here, a bar shaped permanent magnetic and ferromagnetic thin film magneto-resistive elements (hereinafter referred to as MR elements) are used as the position sensor. The details of the position sensor (MR sensor) including the MR elements will be described later.
Referring to FIG. 30, a saw tooth shaped output (4) is obtained by combining outputs (1), (2) and (3) from three MR elements. Since the position is detected by using only those portions where voltage changes largely with respect to the change in phase, accurate interpolation becomes possible.
In the conventional method of detection by the positional sensor devised to improve resolution, outputs from three MR elements have been used, which results in increased number of circuits and increased cost. Further, in this method, when the signal level changes, amplitude of triangular wave also changes, possibly resulting in error in detection.
Further, when a position detecting sensor is to be used for the lens unit, a reference position must be provided near the infinite position for a focus lens, and a reference position must be provided at a telephoto end or a wide end for a zoom lens. In order to detect such reference positions, the magnetic pattern of the magnetized rod may be changed. However, if such method is used, magnetization of the rod is difficult.